In ‘New Work for a Theory of Universals’ and ‘Putnam’s Paradox’ Lewis famously argued that Putnam’s model-theoretic argument against realism fails. His response to Putnam is that there is a real division between natural and non-natural properties out there in the world and that these properties do affect which interpretations are admissible and which are not. In short, realism is saved by appealing to natural properties.
In ‘New Work for a Theory of Universals’ and ‘Putnam’s Paradox’ Lewis credits Gary Merrill for this response to Putnam. See Merrill 1980 ‘The Model-theoretic Argument against Realism’, Philosophy of Science 47, pp. 69-81. Merrill does not endorse the reply but simply asserts that opponents of Putnam should make this move.
In this letter Lewis reacts to Merrill’s reply to Putnam. As is well-known from ‘New Work for a theory of universals’ and ‘Putnam’s Paradox’, Lewis accepts Merrill’s offer and credits him accordingly. Curiously, this letter is dated October 1978. It is one of the earlier letters leading up to Lewis’s adoption of natural properties and his rejection of egalitarianism about properties. This letter predates Lewis’s correspondence and conversations with Armstrong about naturalness. Lewis had to cancel his 1978 (northern) summer trip to Australia. Any discussions with Armstrong that would have risen from Armstrong’s Universals and Scientific Realism occurred after 1978 (Armstrong had planned to give Lewis a copy of U&SR in person in Sydney in 1978). But it is Armstrong who Lewis credits with his conversion to naturalness. An interesting question then arises: why isn’t this letter sufficient motivation for Lewis to adopt an ontology of natural properties?